Resistive device



Nov. 25, 1952 c, 1, GANcl RESISTIVE DEVICE 2 SHEETS-SHEET l Filed Dec.6, 1949 mf v KQ", Mg

Patented Nov. 25, 1952 RESISTIVE DEVICE Charles J. Ganci, BelleroseManor,- N. Y., assignor to Ward Leonard Electric Company, a corporationof New York Application December 6, 1949, Serial No. 131,474

7 Claims. 1

This invention relates to resistive devices and is particularlyapplica-ble to rheostats and the like wherein the resistive conductor orconductors are embedded in insulating material on a flat support whichmay be of circular or polygonal form.

The main object of this invention is to increase the dissipation ofpower of resistive devices and thereby greatly increase their capacity;or in other words, to decrease the size of the resistive devicenecessary for a given power dissipation.

In variable resistive devices, such as rheostats, one of thedifficulties is that the power dissipation or watts capacity is limitedby the hot spot temperature of 250 C. or 300 C., although the rest ofthe device may be considerably under that temperature. One reason forthe occurrence of hot spots is due to only part of the total resistancebeing utilized at times with a comparatively large current whichlocalizes excessive generation of heat in the used portion. Anotherreason is due to the use of tapered resistors which results in someportions of the total resistors having a much greater power dissipationthan other portions, a common ratio of current taper being five to one.A common form of rheostat has an external iron plate as its main supportand although this aids in reducing the hot spot temperatures somewhat,it is of comparatively little help in that respect.

The main feature of this invention is to introduce within the resistivedevice an eflicient heat distributor properly positioned with referenceto the resistive conductors and insulated therefrom. The heatdistributor should preferably be made of metal of good heatconductivity, such as copper, aluminum, and various alloys, theparticular metal or alloy selected being dependent upon the maximumtemperature to which the device is subjected in the making and in itsuse. The distributor may assume various forms and shapes, such as aperforated or solid plate in the form of a disc or otherwise, or a wiremesh or expanded metal material may be used with advantageous results.

The accompanying drawings illustrate preferred embodiments of theinvention. Fig. 1 is a plan View of a circular rheostat plate, partlybroken away to show the ground coat and resistors applied thereon; Fig.2 is a vertical section on the line 2-2 of Fig. 1; Figs. 3, 4, 5, 6 and7 are similar vertical sections showing various modiiications; Fig. 8 isa plan view of a rectangular resistive device with projecting taps andpartly broken away to show the ground coat and resistors connected tothe taps; Fig. 9 is a vertical section on the line 9 9 of Fig. 8; andFigs. 10 and 11 are vertical sections similar to Fig. 9 showingmodifications.

The rheostat shown in Figs. 1 and 2 is provided with the usual circulariron plate I having an upturned edge la. A central opening in the plateis occupied by a metal tube 2 welded or otherwise secured to the plate land serves as a bearing for the mounting of the adjustable contact arm.

A heat distributor 3 in the form of a disc of good heat conducting metalis placed over the base plate and extends over nearly all of its innersurface and is spaced therefrom by small posts or blocks of insulatingmaterial, such as porcelain or glass cord, during the process of making.These spacing insulators may be cemented or otherwise secured to thebase plate or to one side of the heat distributor 3. The distributor isperforated as shown in Fig. 2 with numerous openings 3a distributed overits full extent, as indicated by dotted lines in Fig. l. A ground coat 4of insulating material is then applied over the heat distributor andover the base plate completely embedding and insulating the distributorfrom the base plate. The perforations in the heat distributor permit theinsulating material to pass through and under the distributor so that noopen spaces remain. This ground coat may be of vitreous enamel materialor of various forms of cements or other mixtures and completely embedsand insulates the heat distributor and it provides an insulatingcovering over the distributor of desirable depth for insulating theresistive conductors therefrom. The ground coat is then hardened by ringin a furnace or by drying in accordance with the character of theinsulating material used. The resistors 5, together with their contactbuttons 6 to which they are electrically connected, are next placed inproper relative positions over the top of the matured ground coat. Afurther application of insulating material is then applied over theresistive conductors and lower portions of the contact buttons and thistop layer is then matured by ring or drying according to the nature ofthe insulating material covering the resistors. This results in theresistive elements and the heat distributor being embedded in a solidadherent mass of insulating material 4 as indicated in Fig. 2.

The heat distributor is preferably made of copper which has high heatconductivity and it can also withstand high heating during the making ofthe rheostat and during its use without being distorted or melted,although other metal substances may be used in particular casesaccording to the method of making and use of the device. It serves torapidly and efiiciently transfer the heat developed in any spot orportion of the resistive device to other portions having lowertemperatures. It thereby prevents portions of the device from attainingthe high temperatures which would otherwise be reached and gives a farmore general dissipation of heat from the entire surface of the rheostatthan could otherwise be obtained.

The preferred embedded insulating material is vitreous enamel in theground and cover coats due to its Well known advantages. Where inorganiccements are used for the insulating material, they are treated in aknown manner to produce a hard adherent mass by reacting with water, asin the case of hydraulic cements, or treated with chemicals as in thecase of chemically reacting cements. In some cases where the temperatureof ultimate use is less than 250 C., the insulating'ground coat on themetal base embedding the heat distributor may be formed of granules oforganic or semi-organic resinous material, such as organo-siliconematerials, and subjected to heat and pressure. After applying theresistors and contacts in proper positionthereon, more of such materialis applied and subjected to heat and pressure to form the cover coat.

Fig. 3 is similar to Fig. 2 with the reference characters indicatingcorresponding parts; but in Fig. 3 the perforated metal heat distributor3 is embedded in the insulating material above the resistive elements 5.In this case the copper distributor serves to very emciently distributethe heat. The metal base plate I also aids in such distribution when ofthe customary iron or steel. However, I have found "that'by making thebase plate of copper, a much more efficient distribution of the heat isobtained; and when combined with a perforated copper distributorembedded in the insulation above the resistive conductors, the mostadvantageous result is obtained as regards the greatly increasedcapacity of the device.y

Experiments and test data have shown that the use of a copper base plateand copper heat distributor embedded within a vitreous enamel insulatingmaterial and having a 5 to l taper has resulted in doubling the wattcapacity for the same temperature rise. Thus an S-inch diameter rheostatplate may-replace a l3-inch diameter plate for the same temperature riseor watts capacity. Tests also have shown that when only a portion of theresistive device is incircuit, the load imposed thereon may be verygreat because the heat generated in that portion is conducted rapidly bythe heat distributor to other portions of the device not in circuit.Although the increase in capacity by this improvement is very marked inrheostats having a 5 to 1 current taper, it is of very considerableadvantage in 1 to l current tapers, or equal ohms per step, with allresistance in circuit; and, of course, when only part of the resistorsare in circuit they can be quite heavily loaded because the heat israpidly conducted to cooler portions of the rheostat not in circuit.Copper has proved to be the most desirable metal for the heatdistributor, but other metals, such as aluminum, alloys and other metalsof high heat conductivity may be used. The greater the mass, the greaterthe heat conductivity of the distributor. The perforated plate isdesirable not only for allowing the insulating material to pass throughit and 'fill all spaces 4 during manufacture but because it is moreyieldable under high heating during manufacture or use and less likelyto crack or ake olf the insulating material due to diierences in thecoe'icients of expansion, especially when copper is used with vitreousenamel insulation. Where the insulating material is of cements, ororganic or semi-inorganic resinous material and the maximumtemperatures. during manufacture or use are comparatively low, the metaldistributor need not be perforated as after the resistors are placed onthe ground coat, more insulating material may be applied, then the solidheat distributor placed in position and then the cover coat applied.

Fig. 4 is similar to Figs. 2 and 3 but shows the perforated heatdistributors 3 both above and below the resistive conductors 5. This, ofcourse, secures a more rapid and eiiicient distribution of heat thanobtained in the case of either Figs. 2 or 3, although the advantage isnot proportionally increased for the samesize and mass of thedistributors. However, when it is desired to use distributors of smallthickness and low mass, the structure of Fig. 4 is highly advantageous.

Fig. 5 is similar to Fig. i except the distributors 7 above and belowthe resistors 5 are shown in theform of a wire mesh of metal. Fig. 6 islikewise similar except the distributors 8 are shown in the form ofexpanded metal. These figures illustrate the wire range of choice in thecharacter of the distributors according to particular requirements andconditions.

Fig. 7 shows a structure of a strong and sturdy character adapted towithstand pronounced mechanical shocks in any direction and also havinghigh watt capacity for any given temperature rise. Fig. 7 is similar toFig. 3 except in addition to the distributor 3 located above theresistive conductors, a copper plate 9 forms a lining for the inside ofthe iron plate i. The copper plate is rmly secured to the iron plate Vasby rivets indicated in Fig. 7 in the flat portion and in the upturnededges. The copper plate 9 being a much betterheat conductor than theiro-n plate if, serves to act e'iiciently as a heat distributor incooperation with the plate 3. Good results are obtained even when thedistributor 3 is omitted from this structure. Also in Figs. 2, 4, 5 and6, the distributor below the resistors may sometimes be placed directlyagainst the main base plate I andY good results obtained. In some casesthe base plate itself may be made of copper or of other good heatconducting metal with advantageous results over the use of iron or sheetsteel.

VResistive devices instead of being formed on a metal base are sometimesmade with a base ofV insulating material such as of porcelain or ofvarious organic or inorganic materials. Some forms of resistive devicesinstead of having a movable contact element mounted thereon for changingthe amount of resistance in circuit are merely provided with taps fromwhich variable connections are made for changing the amount ofresistance in circuit. Figs. 8 and 9 show my improvement applied todevices embodying both of these features. These gures show a main basesupport It having a fiat portion of rectangular form with an upwardlyextending edge lila, The insulating material embedding the resistiveconductors 5 may be of any of the kinds already described depending onthe process of making, the character of the insulating base and theintended use of the device. The taps I I connected to the resistiveconductors 5 project above the insulating material for connection to thecontrol circuits, a double row of such taps and resistors being shown.Fig. 9 shows the resistors 5 placed directly on the inside bottom of theinsulating base I9 and shows the heat distributor 3 embedded in theinsulating material 4 above and spaced somewhat from the resistors.

Fig. 10 is similar to Fig. 9 except it shows the heat distributor 3placed directly against the inside botom of the base I and the resistiveconductors 5 are positioned above the distributor and spaced therefromand embedded in the insulating material 4. Fig. 11 is similar to Fig. 10except a heat distributor 1, shown in the form of a metal screen, isembedded in the insulating material 4 and positioned above and spacedfrom the resistors. The structures of Figs. 8 to 11 may be made indiierent ways according to the character of the insulating materialused, the temperatures in the process of making and maximum temperaturesof the intended use, as described with reference to Figs. l to 7.Likewise the heat distributors may be formed in diierent ways, asalready described, of good heat conducting metals such as copper,aluminum, brass and the like depending on the particular requirements.

It is evident that this invention may be embodied in various forms ofstructure and using resistive conductors connected to contact buttons,taps or other forms of terminals. Also various kinds of embeddinginsulating material may be used; and the embedded layers of metal whichform the distributors may be of various forms and be positioned invarious relationships to other parts of the resistive device and spacedfrom the resistors and covering an area approximately co-extensive inextent with that occupied by the resistors.

I claim:

l. A rheostat comprising a plurality of resistive conductors connectedto a plurality of terminals respectively, a supporting base, a heatdistributor of good heat conductivity in the form of a layer of metalspaced from said resistive conductors below said conductors andextending parallel thereto and approximately co-extensive therewith inextent of area, and insulating material embedding said resistiveconductors and portions of said terminals and also embedding and servingas the sole insulation between said heat distributor and said conductorsand terminals.

2. A rheostat comprising a supporting base, a heat distributor in theform of a layer of metal on said base, a plurality of resistiveconductors connected to a plurality of terminals respectively andpositioned above said heat distributor and spaced therefrom andapproximately co-extensive therewith in extent of area, and insulatingmaterial embedding said resistive conductors and portions of saidterminals and also covering and serving as the sole insulation betweensaid heat distributor and said conductor and terminals.

3. A rheostat comprising a supporting base, a heat distributor in theform of a layer of metal on said base, a plurality of resistiveconductors connected to a plurality of terminals respectively andpositioned above said heat distributor and spaced therefrom andapproximately coextensive therewith in extent of area, a second heatdistributor in the form of a layer of metal above and spaced from saidresistive conductors and extending parallel thereto and approximatelyco-extensive therewith in extent of area, and

insulating material embedding said resistive conductors and portions ofsaid terminals and also covering and embedding said heat distributors.

4. A rheostat comprising an outer cup-shaped casing, a hard rigiddisk-shaped insulating material in said casing, a resistor havingresistive segments positioned side by side and spaced apart a distancesubstantially less than their length to form a, compact resistance unitembedded within said insulating material, terminals in electricalcontact with said resistor and projecting outside of said material toprovide external contacts to said resistor, and a metallic heatdistributor of good heat conductivity in the form of a layer embedded insaid insulating material and substantially parallel to and electricallyspaced from said segments, said distributor being approximatelycoextensive in extent of area with said segments to receive heattherefrom and to distribute concentrated heat at one portion of saidresistor throughout the insulating material.

5. A rheostat comprising a rigid insulating member, a resistor havingresistive segments positioned side by side and spaced apart a distancesubstantially less than their length to form a compact resistance unitembedded within said insulating member, end and intermediate terminalsin electrical contact with segments of said resistor and projectingoutside of said insulating member to provide external contacts to saidresistor, and a metallic heat distributor of good heat conductivity inthe form of a layer embedded in said insulating member and substantiallyparallel to and electrically spaced from said segments, said distributorbeing approximately coextensive with and having a high thermal couplingwith said segments to receive heat therefrom and for distributing heatconcentrated at one portion of said resistor.

6. A rheostat comprising a rigid insulating member, a resistor havingresistive segments positioned side by side and spaced apart a distancesubstantially less than their length to form a compact resistance unitembedded within said insulating member, end and intermediate terminalsin electrical contact with segments of said resistor and projectingoutside of said insulating member to provide external contacts to saidresistor, and a metallic heat distributor of good heat conductivity inthe form of a layer embedded in said insulating member and electricallyspaced from said segments, said distributor extending substantiallyparallel to said segments and in a heat exchange relationship with saidresistor to receive heat concentrated at tapped segments for increasingthe rate of dissipation from said tapped segments and spreading the heatin said insulating member for wider dissipation.

7. A rheostat comprising a rigid insulating member, a resistor havingresistive segments positioned side by side and spaced apart a distancesubstantially less than their length to form a compact resistance unitembedded within said insulating member, end and intermediate terminalsin electrical contact with segments of said resistor and projectingoutside of said insulating member on the same side of and at an angle tosaid resistor to provide external contacts to said resistor, and ametallic heat distributor of good heat conductivity in the form of alayer embedded in said insulating member on the projecting side of saidterminals and electrically spaced from said segments, said distributorextending substantially parallel to said segments and in a heat exchangerelationship with said resistor to receive heat concentrated at tappedsegments for increasing the rate of dissipation from said tappedsegments and spreading the heat in said insulating memberifor Widerdissipation.

CHARLES J. GANCI.

' REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Number Name Date Wiegand July 25, 1916Howard Sept. 17, 1940 Mann Nov. 1, 1949 FOREIGN PATENTS Country DateGermany Mar. 3, 1938

